It is known that the rotation of a rotating element relative to a support is rendered possible by means which suppress friction, which is the case for roller bearings or for self-lubricated rings or for rings in an oil bath.
Considering the case of a shaft maintained by two bearings secured to a frame and driven in rotation, it will be seen that there is no connection of the movement between the chassis and the shaft if there is no guidance by the bearings whose object is to maintain the shaft in place.
During its rotation, whether during acceleration, deceleration or constant speed of rotation, the shaft is independent of the frame.
It is known moreover that rotating shafts are subject to flexure which is more or less great to the extent the bearings are spaced, that the shift has a small diameter relative to its extent, that the shaft has a large balancing fault.
This leads particularly to premature wear of the bearings, to vibrations damaging to the connection members to be driven.
In a very simplified vehicle comprising a chassis mounted on four independent wheels and provided with independent propulsion means for the wheels, it is known that as soon as one of the wheels encounters different adherence conditions, this wheel will give rise to at reaction on the chassis.
Thus during its movement, when the vehicle has a wheel bearing on sand for example and the other on macadam, this vehicle has the tendency to lose its stability because the wheel on the sand will have its speed of rotation slowed relative to the other wheels and particularly relative to the wheel situated in the same path.
It is known that the wheels are independent in rotation of each other and particularly that they are not connected to the speed of movement of the chassis relative to the ground on which the vehicle moves.
Thus, one wheel can turn faster than the speed of movement of the chassis relative to the ground, which causes slippage, or slower, which causes blockage.
In the two cases, there is a loss of adherence of the wheel in contact with the ground, with control difficulties that that leads to.
If the vehicle comprises drive means of at least one train of wheels, the problem which arises is the same, but it is masked by other difficulties.
Thus, generally, any vehicle which has a driven wheel train has a differential interposed between the driven wheels such that the vehicle can follow the turning curve in a more progressive manner.
The differential has a very precise role which is that of compensating the difference of rotative speed of the inside wheel with respect to the outside wheel.
It should be noted that the drive shaft drives a large ring gear secured in rotation to a differential housing in which is locked a cross piece receiving satellites mounted freely in rotation which coact with planets secured to the internal end of each of the wheel shafts, themselves mounted freely in rotation thanks to roller bearings.
The outer end of each of the shafts is also mounted freely in rotation relative to the chassis thanks to bearings.
Each free end of the shaft carries one of the motor-driven wheels.
It is also known that the motor-driven wheels are not connected to the chassis during their rotation.
The differential has the role of amplifier-compensator but does not permit any precise connection of the rotative movement of the wheels with the speed of movement of the vehicle relative to the ground.
This absence of connection is responsible for loss of safety of vehicles, as shown in the following example.
An automotive vehicle with two motor-driven wheels and two idling wheels moves over a street paved in macadam.
When the vehicle follows a curve, the differential absorbs the difference of speed of the inner wheel relative to the outer wheel because the planets can displace angularly one relative to the other, by rotation of the satellites.
The couple also presents a different distribution on the two wheels because one adheres better than the other to the macadam.
This spacing of the distribution of the couple is even more flagrant when the vehicle moves over a macadam in a straight line, and the wheels on the same side encountered a sheet of ice for example which therefore modifies the adherence of these wheels.
The couple is distributed in a very unsymmetric way essentially on the motor-driven wheel which has the least adherence.
This wheel skids, which is to say that the speed of rotation of the wheel is greater than the theoretical speed that this wheel should have as a function of the speed of movement of the vehicle relative to the ground, given its development.
As soon as the vehicle wheels rolling on the snow return to the macadam, the coefficient of adherence increases abruptly and there may be a very brief skidding, but in any case the vehicle is unbalanced.
The greater the speed and weight of the vehicle during loss of adherence, the greater the danger.